The present invention relates to a control system for an electromagnetic clutch for an automatic transmission of a motor vehicle.
An automobile provided with a continuously variable belt-drive transmission with an electromagnetic clutch is disclosed in EP-A 151038. The electromagnetic clutch of the transmission is controlled by a control system to provide various operational modes as steady state modes, such as a starting mode of a vehicle, reverse excitation mode, drag mode, and mode of lock-up engagement. One of the modes is selected in accordance with a position of a selector level and driving conditions to control the electromagnetic clutch.
Other prior art discloses control systems which provide transient state modes for various transient states. The transient states therein and herein include states at the changing of ranges, for example changes from a neutral range (N-range) or parking range (P-range) to drive range (D-range), high engine speed drive range (Ds-range) or reverse range (R-range), from the D-range to Ds-range, or reverse range (R-range), from the D-range to Ds-range, and at the changing of modes, for example changes from the starting mode to the lock-up engage mode. Further, the transient state mode includes operations such as releasing and depressing of an accelerator pedal in starting or lock-up engage mode. Herein the term "transient state" is defined to include any of the above. In the transient state mode, clutch torque is temporarily decreased to cause the clutch to slip so as to prevent a shock at the changing operation. The clutch torque is gradually raised thereafter. In another art, the clutch torque is varied with time in accordance with a particular characteristic.
However, the engagement rate of the clutch during driving of the vehicle does not solely depend on the clutch torque but also on the relationship between engine torque and torque-load on the clutch output shaft which depends on running resistance of the vehicle. Thus, it is difficult to cause the clutch to slip at optimum rate only by controlling the clutch torque. Accordingly, in the prior art, there remain driving condition regions where clutch torque does not provide desired engagement rates.